Large-scale database systems using a computer have been constructed in a wide range of fields in recent years. In large-scale database systems, there may occur a need to integrate data scattered over a number of distributed databases.
For example, in a research institute's information system including multiple types of databases for managing various research data acquired by various researchers, there may be a need to integrate scattered pieces of data in order to find out information or knowledge contained in such scattered pieces of data. As another example, there may be a need to integrate different databases that have previously been run by different companies, due to a merger between companies.
In order to integrate such various types of databases distributed on a network, a mechanism for federating databases has been used. This mechanism is a technology that accesses a group of databases existing in a distributed manner rather than a combined database to provide a grouping of database images. This mechanism is one in which individual databases are directly accessed, so it can readily support addition of a new data item. Therefore, the time and/or cost for re-designing or re-constructing the database system can be reduced.